An alternative to Nathan's excellent reply is to use the new labeling tab in QGIS 1.9 (also available in 1.8 but not from the layer properties menu. Instead go to Layer->Labels).

If you scroll down on the label settings tab you will find a Multiple Lines option, where you can specify the character to wrap the line on. This will be helpful if you are concatenating your string using something other than a newline character (as per Nathan's reply) or you have a string that is held in a single field (I often have to contend with slightly 'messy' data from other sources like this).

The other advantage of the multiple lines option is that you can specify alignment and line height too. So you could concatenate using spaces and then align centrally or to the right with a 1.5 line spacing (if that is cartigraphically appealing).

e.g. "Date" || ' ' || "Size" || ' ' || "name"

...with wrapping set to a single space plus align set to 'right' might give you something like: